1. Field of the Invention
The present invention relates to an image processing apparatus for performing color correction in accordance with ambient light. The invention also relates to an image processing method for use in the above apparatus.
2. Related Background Art
Along with the widespread use of color-image products, color images are becoming easier to handle, not only in specific fields, such as computer graphics (CG) design, but also in ordinary small offices. In practice, however, color matching cannot generally be achieved between an image produced on a monitor and the resulting printed image, and it is difficult to check the colors of images on the monitor prior to printing. Attention is now focussed on a color management system directed to solving the above problem.
The color management system is constructed to target, by using a common color space, the elimination of color differences produced by different types of devices. More specifically, this system is constructed based on the idea that all of the colors represented by the same coordinates in the same color space appear alike. Accordingly, color-appearance matching is implemented with the use of this system, i.e., by expressing all of the colors in the same color space and by matching the corresponding coordinates with each other. One of the methods which are currently employed for correcting differences caused by different types of devices is to use the CIE-XYZ color space and its internal description coordinates, i.e., the XYZ tristimulus values.
The environment for observing images is now explained with reference to FIG. 11. FIG. 11 shows that an image 202, which is the same as the image of a printed article 201, is displayed on a monitor 203, and ambient light 204 used for observing the image 202 and the printed image is detected by an ambient-light sensor 206 installed on the monitor 203 or on a printer. The printed image and the image 202 displayed on the CRT are not always observed under the same ambient light, and the ambient light 204 shown in FIG. 11 varies according to environmental conditions. Further, even though the image 202 on the monitor and the printed image appear alike under a certain type of ambient light, they may look completely different with a change in the ambient light. In order to prevent this problem, the above-described color management system is employed, as illustrated in FIG. 10, to predict values (for example, the XYZ values) of each image to be observed under a certain type of ambient light, based on ambient-light information 108 obtained from a sensor 109, and to reproduce the values as faithfully as possible by using profiles 103 and 106 of each device. Thus, color-appearance matching can be achieved.
This method will now be described while referring to FIG. 10. An input image (printed article) is read with a scanner 101, and R1G1B1 values obtained from the scanner 101 are converted into device-independent color signals X1Y1Z1 in a scanner RGBxe2x86x92XYZ converter 102 with the use of the scanner profile 103 in which scanner-characteristic data is stored. The color signals X1Y1Z1 are further converted in a signal converter 104, based on ambient-light information 108 obtained from an ambient-light detecting sensor 109, into color signals X2Y2Z2 of the respective colors to be observed under the ambient light. Then, a monitor profile 106 is used to calculate monitor input values R2G2B2 in an XYZxe2x86x92monitor RGB converter 105.
By employing the aforedescribed method, the colors represented by the same values in the common color space should appear the same. However, it is known that a color displayed on a monitor and the color of a printed article to be observed under illumination appear different to the observer even though both of the colors are represented by the same value. In order to solve this problem, further corrections are required to achieve color-appearance matching performed by visual observations under the above-described environment.
The human eye is considered to recognize all of the colors in relation to the color white. This is further considered by taking an example in which an image displayed on a monitor and a printed article are observed under a certain type of ambient light (environment light).
In the above-described environment, there may be various types of white colors (detectable white colors), such as white colors of a monitor, illumination light, and an image illuminated with illumination light (paper white color). An observer in such an environment detects, considering all of the aforedescribed different types of white colors, a white color which is used as a reference for detecting the other colors. The observer thus observes other colors based on the reference white color. Hence, the following method is considered to match the color appearance: a reference white color is determined in the above-described environment, and the other colors of all of the images are converted based on this reference white color. As an application of the above-described method, a technique disclosed in a thesis (SPIE Publication Vol. 2170 pp170.-181.) is known. This thesis reveals a method for color-appearance matching in which a reference white color is determined based on the white color of a fluorescent lamp and the white color of a monitor under the fluorescent lamp, and color conversion is performed based on the determined reference white color to obtain color-appearance matching.
The aforedescribed method for achieving color-appearance matching by determining a reference white color and converting all of the other colors based on this white color is sufficiently effective when color rendering is high under illumination light. However, any variation in the spectral distribution of illumination light generally changes the resulting color stimulus values, and changes in the color stimulus values further vary depending on the spectral reflectance factor of a reflector. Accordingly, a sufficient degree of color-appearance matching cannot be expected merely by using the method of performing color conversion of all the colors based on a specific white color.
Accordingly, in view of the above background, it is an object of the present invention to achieve color-appearance matching of output images to input images irrespective of ambient light (observing light).
It is another object of the present invention to implement highly-precise color-appearance matching of output images to input images by performing color conversion in consideration of ambient-light characteristics, such as color rendering.
In order to achieve the above-described objects, according to one aspect of the present invention, there is provided an image processing method for performing color conversion of image data representing an input image so as to fulfil color-appearance matching of an output image to be reproduced by an image output device to said input image, the method comprising the steps of: performing color-conversion processing of image data representing an input image obtained from an input device under a certain standard light source into image data dependent on a light source used for observation; performing color-conversion processing, based on a reference white color determined when an observation is made, of the image data dependent on the light source used for observation; and outputting the image data dependent on the reference white color to the image output device.
According to another aspect of the present invention, there is provided an image processing method for performing color conversion of image data representing an input image so as to fulfil color-appearance matching of an output image to be reproduced by an image output device to the input image, the method comprising the step of performing color conversion of image data representing an input image obtained from an input device under a certain standard light source into image data produced in consideration of the characteristics of ambient light and chromatic adaptation which is performed based on a reference white color determined when an observation is made.
According to still another aspect of the present invention, there is provided an image processing apparatus for performing color conversion of image data representing an input image so as to fulfil color-appearance matching of an output image to be reproduced by an image output device to the input image, the apparatus comprising: first color-conversion means for color-converting image data representing an input image obtained from an input device under a certain standard light source into image data dependent on a light source used for observation; second color-conversion means for color-converting the image data dependent on the light source used for observation, based on a reference white color determined when an observation is made; and output means for outputting the image data dependent on the reference white color to the image output device.
According to a further aspect of the present invention, there is provided an image processing apparatus for performing color-signal conversion of image data so as to fulfil color-appearance matching between an image displayed on a monitor and a printed article, the apparatus comprising: setting means for setting the type of ambient light; storage means for storing in advance color-signal processing information on each type of the ambient light; and color-signal conversion means for performing color-signal conversion, based on the color-signal processing information corresponding to each type of the ambient light, of the image data, considering the characteristics of the ambient light and chromatic adaptation performed based on a white color of the monitor.
Further objects, features and advantages of the present invention are apparent from the following description of the preferred embodiments with reference to the attached drawings.